Seed Layer for Multilayer Magnetic Materials

ABSTRACT

A magnetic element is disclosed wherein a composite seed layer such as TaN/Mg enhances perpendicular magnetic anisotropy (PMA) in an overlying magnetic layer that may be a reference layer, free layer, or dipole layer. The first seed layer is selected from one or more of Ta, Zr, Nb, TaN, ZrN, NbN, and Ru. The second seed layer is selected from one or more of Mg, Sr, Ti, Al, V, Hf, B, and Si. A growth promoting layer made of NiCr or an alloy thereof is inserted between the seed layer and magnetic layer. In some embodiments, a first composite seed layer/NiCr stack is formed below the reference layer, and a second composite seed layer/NiCr stack is formed between the free layer and a dipole layer. The magnetic element has thermal stability to at least 400° C.

PRIORITY DATA

The present application is a continuation application of U.S. patentapplication Ser. No. 15/344,618, filed Nov. 7, 2016 and entitled “Seedlayer for Multilayer Magnetic Materials,” which application is adivisional application of U.S. patent application Ser. No. 13/649,327,filed Oct. 11, 2011 and entitled “Seed layer for Multilayer MagneticMaterials,” which applications are hereby incorporated by reference intheir entirety.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. Nos. 8,541,855;and 8,871,365, both assigned to a common assignee and hereinincorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a magnetic element comprising a seedlayer and growth promoting layer with thermal stability to at least 400°C., and that induce a strong (111) crystal structure in an overlyingmultilayer stack thereby improving perpendicular magnetic anisotropy(PMA) including Hc and Hk in the overlying stack.

BACKGROUND

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon CMOS with magnetic tunnel junction (MTJ) technology, is amajor emerging technology that is highly competitive with existingsemiconductor memories such as SRAM, DRAM, and Flash. Similarly,spin-transfer (spin torque or STT) magnetization switching described byC. Slonczewski in “Current driven excitation of magnetic multilayers”,J. Magn. Magn. Mater. V 159, L1-L7 (1996), has stimulated considerableinterest due to its potential application for spintronic devices such asspin-torque MRAM on a gigabit scale.

Both MRAM and STT-MRAM may have a MTJ element based on a tunnelingmagneto-resistance (TMR) effect wherein a stack of layers has aconfiguration in which two ferromagnetic layers are separated by a thinnon-magnetic dielectric layer. The MTJ element is typically formedbetween a bottom electrode such as a first conductive line and a topelectrode, which is a second conductive line at locations where the topelectrode crosses over the bottom electrode. A MTJ stack of layers mayhave a bottom spin valve configuration in which a seed layer, aferromagnetic “reference” layer, a thin tunnel barrier layer, and aferromagnetic “free” layer are sequentially formed on a bottomelectrode. The free layer has a magnetic moment that is either parallelor anti-parallel to the magnetic moment in the reference layer. Thetunnel barrier layer is thin enough that a current through it can beestablished by quantum mechanical tunneling of conduction electrons. Themagnetic moment of the free layer may change in response to externalmagnetic fields and it is the relative orientation of the magneticmoments between the free and reference layers that determines thetunneling current and therefore the resistance of the tunnelingjunction. When a sense current is passed from the top electrode to thebottom electrode in a direction perpendicular to the MTJ layers, a lowerresistance is detected when the magnetization directions of the free andreference layers are in a parallel state (“0” memory state) and a higherresistance is noted when they are in an anti-parallel state or “1”memory state.

As the size of MRAM cells decreases, the use of external magnetic fieldsgenerated by current carrying lines to switch the magnetic momentdirection becomes problematic. Spin torque MRAM was developed to providea robust magnetic switching margin by eliminating the half-selectdisturb issue. Compared with conventional MRAM, spin-torque MRAM has anadvantage in avoiding the half select problem and writing disturbancebetween adjacent cells. The spin-transfer effect arises from the spindependent electron transport properties offerromagnetic-spacer-ferromagnetic multilayers. When a spin-polarizedcurrent transverses a magnetic multilayer in a CPP configuration, thespin angular moment of electrons incident on a ferromagnetic layerinteracts with magnetic moments of the ferromagnetic layer near theinterface between the ferromagnetic and non-magnetic spacer. Throughthis interaction, the electrons transfer a portion of their angularmomentum to the ferromagnetic layer. As a result, spin-polarized currentcan switch the magnetization direction of the ferromagnetic layer if thecurrent density is sufficiently high. The difference between aspin-torque MRAM and a conventional MRAM is only in the write operationmechanism. The read mechanism is the same.

For MRAM and spin-torque MRAM applications, it is often important totake advantage of PMA films with a large and tunable coercivity field(Hc) and anisotropy field (Hk). For example, PMA films may be used as apinned layer, free layer, or dipole (offset compensation) layer in a MTJelement or in PMA media used in magnetic sensors, magnetic data storage,and in other spintronic devices. Furthermore, a critical requirement isthat Hc, Hk, and other properties such as the magnetoresistive (MR)ratio do not deteriorate during processing at elevated temperatures upto 400° C. or higher. In some applications, it is also necessary tolimit the overall thickness of the seed layer and other underlayersbelow the PMA layer to less than about 100 Angstroms, and use onlymaterials that are compatible with device design and high temperatureprocessing requirements.

Materials with PMA are of particular importance for magnetic andmagnetic-optic recording applications. Spintronic devices withperpendicular magnetic anisotropy have an advantage over MRAM devicesbased on in-plane anisotropy in that they can satisfy the thermalstability requirement and have a low switching current density but alsohave no limit of cell aspect ratio. As a result, spin valve structuresbased on PMA are capable of scaling for higher packing density, which isone of the key challenges for future MRAM applications and spintronicdevices.

When the size of a memory cell is reduced, much larger magneticanisotropy is required because the thermal stability factor isproportional to the volume of the memory cell. Generally, PMA materialshave magnetic anisotropy larger than that of conventional in-plane softmagnetic materials such as NiFe or CoFeB. Thus, magnetic devices withPMA are advantageous for achieving a low switching current and highthermal stability.

Several PMA material systems have been proposed and utilized to datesuch as multilayers of Pt/Fe, Pd/Co, and Ni/Co, and ordered (e.g., L10structures) as well as unordered alloys but there is still a need forimprovement in Hc, Hk, temperature stability, and materialcompatibility. Among the PMA material systems studied, a Ni/Comultilayer is one of the more promising candidates because of its largepotential Hc and Hk, good stability at high anneal temperatures, andpotential compatibility with other materials used in magnetic devices.However, Ni/Co multilayers and the like typically require a thick seedlayer to induce high PMA. Seed layers thicker than about 100 Angstromswill require thinning a different layer in the spin valve structure tomaintain a certain minimum thickness for the MTJ element that can easilylead to performance degradation.

An improved seed layer is still needed that is thin enough to becompatible with spintronic devices, can induce greater PMA in overlyingCo/Ni multilayer structures or CoFeB layers, and is compatible with thedesign and processing requirements of magnetic devices.

SUMMARY

One objective of the present disclosure is to provide an underlayerstack including a seed layer that will promote greater Hk and Hc in anoverlying PMA layer than previously realized and with thermal stabilityup to 400° C. process temperatures.

A second objective of the present disclosure is to provide a materialset for a high PMA structure according to the first objective that iscompatible with other layers in the magnetic device and has anunderlayer thickness of about 100 Angstroms or less.

According to one embodiment, these objectives are achieved with amagnetic element that is a MTJ with a bottom spin valve configuration inwhich a seed layer, PMA reference layer, tunnel barrier, free layer, andcapping layer are sequentially formed on a substrate. The seed layer(underlayer) is preferably a stack with a lower layer made of one ormore of Ta, Zr, Nb, TaN, ZrN, NbN, or Ru, and an upper layer that is oneor more of Mg, Sr, Al, Ti, V, Hf, B, Si, MgZr, or MgNb. However, theseed layer may also be a single layer made of an alloy that combines oneelement or alloy from the aforementioned materials that may serve as alower layer and one element or alloy from the aforementioned materialsthat may serve as an upper layer. The seed layer structure induces astrong (111) texture in the overlying PMA reference layer and promotes asmooth surface on overlying layers. Preferably, the reference layer hasa Co_(X)Fe_(Y)Ni_(Z)B_(V) composition where y>x+z, and v is from about15 to 40 atomic % in the alloy. The tunnel barrier layer is preferablyMgO.

In a second embodiment, the seed layer may be TaN, or a single layeralloy as described above, or a bilayer stack from the previousembodiment. The magnetic element further comprises a growth promotinglayer between the seed layer and reference layer. The growth promotinglayer is one of NiCr or NiCrM where M is one of Si, V, Fe, Mg, Zr, Nb,Ta, Ti, Cu and Mo that enhances PMA in the overlying reference layer andinduces a strong (111) crystal structure therein. The growth promotinglayer has a Cr content between 35 and 45 atomic %, and preferably 40atomic %, and a thickness from 1 to 30 nm and preferably about 5 nm. Inone aspect, the reference layer is a multilayer represented by(Ni/Co)_(n) where n is the lamination number that is from 2 to 30, eachNi layer has a thickness of about 6 Angstroms, and each Co layer has athickness of about 2.5 Angstroms. Optionally, Ni may be replaced by NiFeor NiCo, and Co may be replaced by CoFe in the laminated stack. In analternative embodiment, the reference layer may be any face centeredcubic (FCC) magnetic layer such as (Co/Pt)_(n), (Co/Pd)_(n),(Fe/Pt)_(n), or (Fe/Pd)_(n) having PMA. The magnetic element may alsoinclude a transitional layer made of CoFeB, CoFe, or Co between the(111) crystalline reference layer and the tunnel barrier layer that ispreferably MgO with a (100) texture. The free layer may be comprised ofCoFeB, CoFe, or a combination thereof. Thus, a high MR ratio is achievedtogether with enhanced PMA in the reference layer to enable greaterthermal stability in the magnetic element.

According to a third embodiment, the reference layer in the secondembodiment is modified to a synthetic anti-ferromagnetic (SAF)configuration wherein a non-magnetic coupling layer such as Ru issandwiched between two laminated (Ni/Co) stacks, for example. To furtherenhance RKKY coupling, there may be a Co dusting layer on either side ofthe Ru coupling layer to give a (Ni/Co)_(n)/Co/Ru/Co/(Ni/Co)_(p)configuration where p is the lamination number of the second laminatedstack having PMA character. In a fourth embodiment, a non-magnetic layersuch as Ta is inserted between the upper (Ni/Co)_(p) portion of the SAFreference layer and a transition layer. As a result, the magneticelement has a stack represented by seed layer/growth promotinglayer/(Ni/Co)_(n)/Co/Ru/Co/(Ni/Co)_(p)/Ta/CoFeB/Co/MgO/freelayer/capping layer wherein CoFeB/Co is the transition layer. In anotherembodiment, the magnetic element may have a top spin valve structurewherein a seed layer, PMA free layer, tunnel barrier, reference layer,and capping layer are sequentially formed on a substrate.

The present disclosure also encompasses an embodiment in which a dipolelayer with a laminated PMA stack as defined in previous embodiments isused to provide an offset field to an adjacent free layer. The MTJ has astack represented by seed layer/reference layer/tunnel barrier/freelayer/underlayer/dipole layer/capping layer. The underlayer may have aseed layer/growth promoting layer configuration.

Once all the layers in the MTJ stack are laid down, a high temperatureannealing of about 350° C. may be employed to increase the PMA withinthe laminated reference layer stack, free layer, or dipole layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a magnetic element having aseed layer/PMA layer stack in a bottom spin valve configurationaccording to a first embodiment of the present disclosure.

FIG. 2 is a cross-sectional view showing a magnetic element having aseed layer/growth promoting layer/PMA layer stack in a bottom spin valveconfiguration according to a second embodiment of the presentdisclosure.

FIG. 3 is a cross-sectional view showing a magnetic element having aseed layer/growth promoting layer/PMA layer stack in a bottom spin valveconfiguration according to a third embodiment of the present disclosure.

FIG. 4 is a cross-sectional view showing a magnetic element having aseed layer/growth promoting layer/PMA layer stack in a bottom spin valveconfiguration according to a fourth embodiment of the presentdisclosure.

FIG. 5 is a cross-sectional view showing a magnetic element having aseed layer/growth promoting layer/PMA layer stack in a top spin valveconfiguration according to a fifth embodiment of the present disclosure.

FIG. 6 is a cross-sectional view of a magnetic element according to asixth embodiment wherein a dipole layer serves as an offset compensationlayer.

FIGS. 7a-7h are a series of graphs that illustrate magnetic propertiesmeasured perpendicular to the film plane for CoFeB layers grown ondifferent underlayers annealed at 300° C. (dashed line) and then postannealed at 400° C. (solid line).

FIG. 8 is a plot illustrating magnetic properties measured perpendicularto the film plane for Co/Ni multilayers grown on a non-optimalunderlayer (dashed line), and on an optimized underlayer (solid line)according to an embodiment of the present disclosure.

FIG. 9 is a plot illustrating M(H) loop measurements in a directionperpendicular to the film plane for a magnetic element grown on a lessthan optimal underlayer (dashed line) and on an optimized underlayeraccording to an embodiment described herein.

DETAILED DESCRIPTION

The present disclosure is a magnetic element with an underlayer/PMAlayer configuration wherein the underlayer is a seed layer or seedlayer/growth promoting layer stack that induces a strong (111)crystalline structure in an overlying (Ni/Co)_(n) multilayer or CoFeNiBlayer thereby enhancing PMA in the overlying layer that may be a pinnedlayer, reference layer, dipole (offset compensation) layer, or freelayer. The terms “seed layer” or “growth promoting layer” may be usedinterchangeably with the term “underlayer” in the exemplary embodiments,and (A1/A2)_(n) and (A2/A1)_(n) are used interchangeably when referringto stack of “n” laminations wherein A1 is a first magnetic element oralloy, and A2 is a second magnetic element or alloy. Although onlybottom and top spin valve structures are depicted in the drawings, thepresent disclosure also encompasses dual spin valves as appreciated bythose skilled in the art. Moreover, the underlayer/PMA layerconfiguration described herein may be incorporated in one or more of areference layer, free layer, dipole layer, or pinned layer in a MRAM,spin-torque-MRAM, domain wall motion device, and in other spintronicdevices.

A key feature of the present disclosure is an underlayer/PMA layer stackwherein the underlayer enhances properties such as Hc and Hk in theoverlying PMA layer. In particular, PMA character is enhanced in the PMAlayer by the presence of an appropriate seed layer (underlayer) having a(111) texture. Ideally, the underlayer has a composition which iscompatible with other materials in a magnetic element, is thermallystable to process temperatures up to 400° C., and is thin enough so asnot to adversely affect other magnetic properties of the magneticelement.

Referring to FIG. 1, a first embodiment of the present disclosure isdescribed with respect to a magnetic element with a bottom spin valveconfiguration that is formed on a substrate 10. The substrate 10 may bea bottom electrode layer, for example, made of Ta or other conductivelayers. Each of the layers in the MTJ is formed in an (x, y) plane andwith a thickness in a z-axis direction. Substrate 10 may be formed on asubstructure (not shown) that includes dielectric and conductive layersas well as transistors and other devices. A key feature is the seedlayer 22 formed on substrate 10. Preferably, the seed layer 22 is acomposite with a lower layer 20 made of one or more of Ta, Zr, Nb, TaN,ZrN, NbN, and Ru with a thickness from 10 to 100 Angstroms. Lower layer20 promotes a uniform thickness, (111) crystal structure, and smooth topsurfaces in overlying layers. Top surface in this context is a surfacefacing away from substrate 10. Seed layer 22 also has an upper layer 21with a thickness of 2 to 20 Angstroms and is made of one or more of Mg,Sr, Ti, Al, V, Hf, B, Si, or an alloy of Mg with Zr or Nb. The (111)texture of upper layer 21 is advantageously used to induce a (111)texture in an overlying magnetic layer 23. In the exemplary embodiment,magnetic layer 23 is shown as a reference layer but the presentdisclosure anticipates that the magnetic layer may be a free layer,dipole layer, or pinned layer rather than a reference layer as presentedin subsequent embodiments.

In an alternative embodiment wherein seed layer 22 consists of a singlelayer, the seed layer has a thickness from 10 to 100 Angstroms and ispreferably an alloy that includes one of Ta, Zr, Nb, TaN, ZrN, NbN, andRu, and one of Mg, Sr, Ti, Al, V, Hf, B, Si, MgZr, and MgNb. We havedisclosed the use of a Hf/NiCr seed layer for an overlying Ni/Comultilayer in related U.S. Pat. No. 8,541,855.

In one aspect, magnetic layer 23 is comprised of Co_(X)Fe_(Y)Ni_(Z)B_(V)where y>x+z, v is from about 15 to 40 atomic % in the alloy, andv+x+y+z=100 atomic %. When z=0, this embodiment encompasses a CoFeBcomposition. The magnetic layer has a bottom surface that contacts seedlayer 22 and an upper surface which adjoins tunnel barrier layer 33. Ina preferred embodiment, the tunnel barrier layer is an oxide such as MgOthat promotes interfacial perpendicular magnetic anisotropy in themagnetic layer 23. When the thickness of the magnetic layer is about 20to 25 Angstroms or less, then the interfacial perpendicular magneticanisotropy may be sufficiently large to overcome the in-planemagnetization and establish PMA within the magnetic layer. In this case,the seed layer 22 enhances the PMA property within magnetic layer 23.

The present disclosure also encompasses an embodiment wherein themagnetic layer 23 has intrinsic PMA and the seed layer is employed toenhance the PMA property in the overlying magnetic layer. For example,magnetic layer 23 may be comprised of (Ni/Co)_(n), (CoFe/Ni)_(n),(Co/NiFe)_(n), (Co/NiCo)_(n), (CoFe/NiFe)_(n), or (CoFe/NiCo)_(n)laminates, for example, where n is between 2 and 30, and preferably 4 to10. Furthermore, a face centered cubic (FCC) magnetic layer such as(Co/Pt)_(n), (Co/Pd)_(n), (Fe/Pt)_(n), or (Fe/Pd)_(n) may be selected asthe PMA magnetic layer. In yet another embodiment, the magnetic layerwith PMA is an unordered alloy or a L10 ordered alloy of the form MTwherein M is Rh, Pd, Pt, Ir, or an alloy thereof, and T is Fe, Co, Ni oralloy thereof. Furthermore, the MT alloy may be doped with B to give aboron content up to 40 atomic %.

As mentioned previously, tunnel barrier layer 33 is preferably made ofMgO although other oxides such as AlOx, TiOx, and ZnOx may be employedas the tunnel barrier. A MgO tunnel barrier layer may be fabricated bydepositing a first Mg layer on the magnetic layer 23, then performing anatural oxidation (NOX) or radical oxidation (ROX) process, and finallydepositing a second Mg layer on the oxidized first Mg layer. During asubsequent annealing process, the second Mg layer is oxidized to afforda substantially uniform MgO layer. If a low RA (resistance×area) valueis desired, a NOX process is generally preferred or the thickness of thetunnel barrier 33 may be reduced as appreciated by those skilled in theart.

According to the exemplary embodiment, a free layer 34 is formed on thetunnel barrier layer 33 and may be made of one or more of CoFeB andCoFe, or another free layer material which produces a combination ofhigh MR ratio, good switching property, and low magnetostriction. Thoseskilled in the art will recognize that the free layer 34 and magneticlayer 23 positions may be switched to form a top spin valve structure asexplained later with regard to FIG. 5.

The uppermost layer in the spin valve stack is a capping layer 35 thatmay be a single layer of Ta or Ru, a composite with a Ru/Ta/Ruconfiguration, or other capping layer materials used in the art.According to one embodiment, the capping layer may be an oxide such asMgO, MgTaOx, or AlOx, or a composite with a lower oxide or nitride layerand an upper Ru or Ta layer. An oxide that serves as the capping layeror as the lower layer in the capping layer may be advantageously used topromote PMA in free layer 34 through an oxide/magnetic materialinterfacial interaction.

In a second embodiment depicted in FIG. 2, a seed layer/growth promotinglayer stack is employed to enhance PMA in an overlying reference layerthat is a stack having a laminated structure with an (A1/A2)_(n)configuration where n is the lamination number, A1 is a first magneticelement or alloy and A2 is a second magnetic element or alloy.Previously, we have fabricated (A1/A2)_(n) reference layers that aregrown on a NiCr or NiFeCr seed layer. However, NiCr based seed layersoften generate a certain amount of roughness in overlying layers due tograin growth. Non-uniformity is an issue for magnetic multilayers suchas (Ni/Co)_(n) when they are combined with functional layers whoseproperties highly depend on the degree of roughness at variousinterfaces in a magnetic element. For example, a smooth top surface in alaminated reference layer promotes a more uniform top surface insubsequently deposited layers including a RKKY coupling layer and asecond reference layer in a synthetic anti-ferromagnetic (SAF) design,and a smoother top surface in an overlying tunnel barrier layer.

We have surprisingly found that the combination of a TaN/Mg seed layer22 and an overlying growth promoting layer 24 made of NiCr or a NiCralloy generates enhanced PMA in a subsequently deposited laminatedreference layer 25. The growth promoting layer has a thickness from 10to 100 Angstroms. The NiCr alloy contains at least one element selectedfrom Si, V, Fe, Mg, Zr, Nb, Ta, Ti, Cu, and Mo. The resulting magneticelement (MTJ) has high thermal stability to at least 400° C. andimproved uniformity (low roughness) in the individual layers, especiallyin laminated reference layer 25, tunnel barrier 33, and free layer 34.In another aspect, the one or more materials selected for the seed layerare previously described in the first embodiment. Thus, a bilayer schemewith lower layer 20 and upper layer 21, or a single layer with athickness from 10 to 100 Angstroms and made of an alloy of one elementor alloy from lower layer 20 and one element or alloy from upper layer21 may be employed as the seed layer 22 in the second embodiment.

Laminated reference layer 25 may be comprised of two metals, a metal andan alloy, or two alloys having an (A1/A2)_(n) configuration where Al isa first metal or alloy selected from one or more of Co, Ni, and Fe thatmay be doped with boron up to 50 atomic %, A2 is a second metal or alloyselected from one or more of Co, Fe, Ni, Ru, Si, Ir, Rh, V, Mg, NiCr,Pt, and Pd, where “n” is the number of laminates in the (A1/A2)_(n)stack. It should be understood that the laminated (A1/A2)_(n) stack hasintrinsic PMA and the seed layer 22 is employed to enhance the PMAproperty. Thus, the reference layer 25 may be comprised of (Ni/Co)_(n),(CoFe/Ni)_(n), (Co/NiFe)_(n), (Co/NiCo)_(n), (CoFe/NiFe)_(n), or(CoFe/NiCo)_(n) laminates, for example, where n is between 2 and 30, andpreferably 4 to 10. Note that an (A2/A1)_(n) configuration provides thesame performance as an (A1/A2)_(n) stack and is considered to beequivalent to an (A1/A2)_(n) laminated stack for the purposes of thisdisclosure. The growth promoting layer/laminated PMA stack describedherein may be deposited at room temperature or at an elevatedtemperature up to 450° C. and can be used as deposited or after anadditional anneal. Alternatively, the reference layer may be any facecentered cubic (FCC) magnetic layer having perpendicular magneticanisotropy including but not limited to (Co/Pt)_(n), (Co/Pd)_(n),(Fe/Pt)_(n), and (Fe/Pd)_(n).

Optionally, a transition layer 32 made of CoFeB, CoFe, Co or acombination thereof is provided between the reference layer and thetunnel barrier layer. According to one embodiment, PMA within transitionlayer 32 is achieved as a result of the interface with a metal oxidelayer in tunnel barrier 33 that leads to a significant amount ofinterfacial perpendicular anisotropy. Moreover, the magnetic moments oflayers 25, 32 are aligned in the same direction along the z-axis. Layer32 serves as a transition layer between the (111) texture in laminatedreference layer 25 and a (100) texture in tunnel barrier 33 and may alsoenhance the magnetoresistive (MR) ratio of the magnetic element. As thetransition layer thickness becomes closer to 6 Angstroms, PMA characteris maximized, and as layer 32 thickness approaches 14 Angstroms, MRratio is increased. Therefore, the thickness of the transition layer maybe adjusted between 6 and 14 Angstroms to tune both PMA magnitude and MRratio. A Co transition layer is less susceptible to oxidation than aCoFe or CoFeB layer and preferably contacts the tunnel barrier layerwhen layer 32 has a CoFeB/Co or CoFe/Co configuration.

Free layer 34 and capping layer 35 are retained from the firstembodiment. However, the free layer may have a laminated structure suchas (CoFe/Ni)_(m), (Co/NiFe)_(m), (Co/NiCo)_(m), (CoFe/NiFe)_(m), or(CoFe/NiCo)_(m), for example, where m is a lamination number preferablybetween 4 and 10, and m≤n in the reference layer laminate. In analternative embodiment, the free layer may have a CoFeB/IL/CoFeBconfiguration where IL is a non-magnetic insertion layer such as Ta thatserves as a moment diluting layer.

In a third embodiment illustrated in FIG. 3, a composite reference layer30 conforms to a synthetic anti-ferromagnetic (SAF) configurationwherein two laminated PMA stacks 25, 29 represented by (A1/A2)_(n) and(A1/A2)_(p), respectively, as described previously are separated by anon-magnetic spacer 27 thereby providing anti-ferromagnetic (RKKY)coupling between the two laminated stacks. Optionally, one or both ofthe PMA stacks may comprise a FCC laminate or an ordered L10 alloy asdescribed earlier. The number of laminations “p” in the second laminatedPMA stack 29 is between 2 and 30, and preferably from 4 to 10, and isnot necessarily equal to “n” in the first laminated PMA stack 25. Inthis embodiment, the non-magnetic spacer is preferably Ru with athickness of 3 to 20 angstroms. To further promote RKKY coupling andenhance Hc and Hk in the adjoining laminated PMA layers, a first dustinglayer 26 such as Co with a thickness of 1 to 10 Angstroms may beinserted between laminated PMA stack 25 and spacer 27, and a seconddusting layer 28 with a similar thickness may be inserted between spacer27 and laminated PMA stack 29 to give a (A1/A2)_(n)/Co/Ru/Co/(A1/A2)_(p)configuration for composite reference layer 30. Thus, the (A1/A2)_(n)stack is considered equivalent to an AP2 layer and the (A1/A2)_(p) stackfunctions like an AP1 layer in a conventional AP2/Ru/AP1 reference layerconfiguration. In related U.S. Pat. No. 8,871,365, we disclosed the useof a Co/Ru/Co spacer to promote RKKY coupling between two magneticlayers.

In a fourth embodiment shown in FIG. 4, an insertion layer 31 such as Tamay be included between the laminated PMA stack 29 and transition layer32 to prevent premature crystallization of an amorphous CoFeB transitionlayer before the tunnel barrier is deposited. Otherwise, all layers fromthe third embodiment are retained in the fourth embodiment. A Tainsertion layer is preferably about 0.5 to 3 Angstroms thick and morepreferably 1.5 Angstroms thick. As a result, crystallization of anamorphous CoFeB transition layer during a subsequent annealing step isdriven by the (100) MgO tunnel barrier and the upper portion of theCoFeB transition layer adjoining the tunnel barrier crystallizes in a(100) state to maximize the MR ratio in the MTJ. It should be understoodthat a lower portion of a CoFeB transition layer which adjoins theinsertion layer will have a (111) crystal structure because of closeproximity to the reference layer or may remain amorphous, but thethickness of this lower portion is believed to be less than the upperportion with a (100) texture.

Referring to FIG. 5, a fifth embodiment is depicted that shows amagnetic element wherein the seed layer/growth promoting layer stack ofthe present disclosure is employed to enhance PMA in an overlying freelayer 34. In other words, the underlayer stack defined herein is notlimited to enhancing PMA in an overlying reference layer but may be usedto improve PMA in a free layer or dipole layer as well. Preferably, themagnetic element in this embodiment has a top spin valve structure inwhich a seed layer, growth promoting layer, free layer, tunnel barrier,reference layer, and a capping layer are sequentially formed onsubstrate 10. The seed layer 22 and growth promoting layer 24 have thesame composition and function as described previously. Reference layer23 was also described earlier with regard to FIG. 1. According to oneembodiment, the free layer 34, tunnel barrier 33, and capping layer 35may be made of the same material as described in previous embodiments.In other words, free layer 34 may be comprised of CoFeB, CoFe, or acombination thereof. However, the present disclosure also encompasses afree layer that is a laminated stack with PMA having a plurality “r” of(A1/A2) layers, or is a face centered cubic (FCC) magnetic layer havingperpendicular magnetic anisotropy including but not limited to(Co/Pt)_(r), (Co/Pd)_(r), (Fe/Pt)_(r), or (Fe/Pd)_(r), or is an orderedL10 alloy. In this case, lamination number “r” for the (A1/A2)_(r) stackor the FCC laminated stack is from 1 to 10.

Referring to FIG. 6, a sixth embodiment of the present invention isillustrated and depicts a magnetic element having a bottom spin valvestructure wherein the seed layer/growth promoting layer configurationserves as an underlayer for a dipole layer. The dipole layer reduces theoffset of the minor switching loop of the free layer caused by a dipolefield from the reference layer. Thus, the magnetic element structure ofthe second embodiment (FIG. 2) is modified by inserting a second seedlayer 42, a second growth promoting layer 44, and the dipole layer 39that are sequentially formed on a top surface of free layer 34. Thesecond seed layer and second growth promoting layer may have the samecomposition as seed layer 22 and growth promoting layer 24,respectively, that were described in an earlier embodiment. Preferably,there is a thin reference layer 25 with a thickness from 5 to 15Angstroms that is formed on a stack made of a lower first seed layer 20and an upper second seed layer 21. A growth promoting layer (not shown)may be formed between the seed layer 22 and reference layer 25. Freelayer 34 and capping layer 35 were previously described with regard tothe first embodiment. In one aspect, a CoFeB/MgO/CoFeB referencelayer/tunnel barrier/free layer stack may be employed to provide a highMR ratio. Optionally, reference layer 25 may be a laminated (A1/A2)_(n)structure, or an ordered L10 alloy. In another aspect, a singlelaminated (A1/A2)_(n) stack may be replaced by a composite referencelayer 30 described in previous embodiments.

A key feature is the stack of layers formed between the free layer andcapping layer. The second seed layer 42, preferably made of Ta, contactsa top surface of free layer 34 and getters oxygen from the free layer. Asecond growth promoting layer 44 contacts a bottom surface of dipolelayer 39 wherein the dipole layer is preferably made of a material thatexhibits intrinsic PMA such as (A1/A2)_(q) laminated structure where qis from 2 to 30, or is a L10 ordered material as described earlier. Inone aspect, free layer 34 may be sufficiently thin (6 to 15 Angstroms)to have significant interfacial perpendicular anisotropy that dominatesan in-plane shape anisotropy field such that a magnetizationperpendicular to the plane of the free layer is established. Interfacialperpendicular anisotropy is a result of the interface between a bottomsurface of free layer and a top surface of tunnel barrier 33 which ispreferably MgO. When the free layer has PMA, the magnetizationdirections of the free layer and dipole layer 39 are preferably alignedin the same direction.

It should be understood that the modifications described in the thirdand fourth embodiments may be included in the sixth embodiment. In otherwords, one or both of a transition layer 32 and Ta insertion layer 31may be formed between the reference layer 25 (or 30) and the tunnelbarrier layer 33.

All of the layers in the magnetic elements described herein may beformed in a sputter deposition system such as an Anelva C-7100 thin filmsputtering system or the like which typically includes three physicalvapor deposition (PVD) chambers each having 5 targets, an oxidationchamber, and a sputter etching chamber. At least one of the PVD chambersis capable of co-sputtering to deposit an alloy from two targets made ofsingle elements. Typically, the sputter deposition process involves anargon sputter gas with ultra-high vacuum and the targets are made ofmetal or alloys to be deposited on a substrate. All of the MTJ layersmay be formed after a single pump down of the sputter system to enhancethroughput.

The present disclosure also encompasses an annealing step after alllayers in the magnetic element have been deposited. The anneal stepcomprises applying a temperature between 300° C. and 400° C. for aperiod of 30 minutes to 5 hours using a conventional oven, or for only afew seconds when a rapid thermal anneal oven is employed. No appliedmagnetic field is necessary during annealing.

Once all the layers in the magnetic elements are formed, the spin valvestack is patterned into an oval, circular, or other shapes from atop-down perspective along the z-axis by a well known photoresistpatterning and reactive ion or ion beam etch transfer sequence.Thereafter, an insulation layer (not shown) may be deposited on thesubstrate 10 followed by a planarization step to make the insulationlayer coplanar with the capping layer 35. Next, a top electrode (notshown) may be formed on the capping layer.

Example 1

An experiment was performed to demonstrate the advantage of a seed layerformed according to an embodiment of the present disclosure in improvingmagnetic properties in an overlying CoFeB reference layer. Anunpatterned spin valve stack comprised of a seed layer, a 9 Angstromthick Co₂₀Fe₅₀B₃₀ reference layer, MgO tunnel barrier layer, a 3Angstrom thick CoFeB free layer, and a Ta20/Ru50 capping layer that weresequentially formed on a Si/SiO₂ substrate was fabricated. All layerswere deposited at room temperature and annealed at 300° C. for 10minutes and then annealed at 400° C. for 30 minutes. In each graph, thesingle layer or bilayer stack listed in the lower right corner refers tothe seed layer composition. The single layers of Ta, TaN, and Zr in FIG.7b , FIG. 7c , and FIG. 7e , respectively, are each 50 Angstroms thick.FIG. 7a is a reference example where no seed layer is formed on the SiO₂substrate. In a bilayer configuration, the lower TaN layer is 50Angstroms thick, and the upper Mg, Mg:Zr (1:1) alloy, or Ta layer inFIG. 7g , FIG. 7f , and FIG. 7d , respectively, is 10 Angstroms thick. AKerr magnetometer was used to generate the data shown in FIGS. 7a-7h . Ahigher amplitude in a curve means a higher PMA is achieved. Thus, aTaN/Mg seed layer according to an embodiment described herein leads tohigher PMA than a Ta seed layer after a final anneal of 400° C. Withregard to the bilayer configurations, the TaN/Mg and TaN/Mg:Zr examplesare considered to be more thermally stable than a TaN/Ta seed layersince the latter loses some amplitude at 400° C. compared with a 300° C.anneal while the seed layers of the present disclosure retainessentially all of their PMA amplitude after the anneal temperature isincreased to 400° C. from 300° C.

Example 2

To further demonstrate the benefits of the present disclosure accordingto an embodiment where the composite seed layer promotes enhanced PMA inan overlying laminated PMA layer that serves as a reference layer, astack of layers was fabricated according to the following configurationwhere the number following each layer is the thickness in Angstroms:Si/SiO₂/TaN50/Mg10/NiCr50/(Co2.5/Ni6)₁₀/Co4/Ru4/Co4/(Co2.5/Ni6)₆/Ta1.5/Co₂₀Fe₆₀B₂₀6/Co4/MgO/Co₂₀Fe₅₀B₃₀3/Ta20/Ru. In the aforementioned structure, Si/SiO₂ isthe substrate, TaN/Mg is a composite seed layer, NiCr is the growthpromoting layer, (Co/Ni)₁₀/Co/Ru/Co/(Co/Ni)₆ is a composite referencelayer, Ta is an insertion layer, CoFeB/Co is the transitional magneticlayer adjoining a MgO tunnel barrier, Co₂₀Fe₅₀B₃₀ is place holder for afree layer since the layer is too thin to be magnetic, and Ta/Ru is thecapping layer. A second stack of layers was fabricated according to adesign previously practiced by the inventors and serves as a referencesample. The second stack differs from the first stack only in theunderlayer structure where TaN/Mg/NiCr is replaced by Ta50/NiCr50.

Referring to FIG. 8, a M-H loop measurement is illustrated for the twostacks of layers as described above that were annealed at 400° C. for 30minutes. Magnetic properties are measured perpendicular to the filmplane of the layers in each stack. Curve 60 for the first stack with aTaN/Mg/NiCr underlayer configuration shows a higher saturation field(>6000 Oe) than realized for the second stack (curve 61) where films aregrown on the inadequate Ta/NiCr underlayer. Thus, higher PMA is realizedin the reference layer and thermal stability to at least 400° C. isachieved when an improved underlayer according to an embodiment of thepresent disclosure is employed.

Example 3

The previous experiment was repeated with the same film stacks exceptthe thickness of the Co₂₀Fe₅₀B₃₀ layer was increased to 12 Angstromssuch that the film has a magnetic property and functions as a freelayer. As a result, the first and second stacks are considered to bemagnetic tunnel junctions (MTJs) suitable for spin-torque MRAM devices.The first stack or MTJ1 now has the following configuration:Si/SiO₂/TaN50/Mg10/NiCr50/(Co2.5/Ni6)₁₀/Co4/Ru4/Co4/(Co2.5/Ni6)₆/Ta1.5/Co₂₀Fe₆₀B₂₀6/Co4/MgO/Co₂₀Fe₅₀B₃₀12/Ta20/Ru. The second stack or MTJ2 has the samelayers except for the underlayer structure, which is Ta50/NiCr50 insteadof TaN/Mg/NiCr in MTJ 1.

Referring to FIG. 9, a M-H loop measurement is illustrated for MTJ1(solid line) and MTJ2 (dashed line) after they were annealed at 400° C.for 30 minutes. Magnetic properties are measured perpendicular to thefilm plane of the layers in each stack. Curve 62 a for MTJ1 shows ahigher saturation field (>5000 Oe) than realized for the MTJ2 (curve 63a) where films are grown on the inadequate Ta/NiCr underlayer.Accordingly, higher PMA is realized in the reference layer when animproved underlayer according to an embodiment of the present disclosureis employed. Thermal stability to at least 400° C. is also demonstrated.Note that steps 62 b, 63 b in the M-H loops indicate the free layer ineach MTJ switches independently of the corresponding reference layer.

The disclosed underlayer structure with various embodiments including asingle layer (alloy), a dual layer, or a single layer or dual layer withan overlying growth promoting layer offer improved magnetic propertiesin a PMA layer formed on the underlayer. Improvements include enhancedPMA, reduced roughness, and better compatibility with high temperatureprocessing up to 400° C. or higher. The embodiments described herein maybe employed with traditional PMA materials and also materials such asCoFeB that have in-plane magnetization but can be induced to haveperpendicular magnetic anisotropy. Moreover, a first underlayer of thepresent disclosure may be used to enhance PMA in an overlying referencelayer while a second underlayer as defined herein may be employed toenhance PMA in an overlying dipole layer within the same magneticelement.

While this disclosure has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this disclosure.

What is claimed is:
 1. A magnetic element, comprising: a seed layerincluding a first material and a second material different from thefirst material; a growth promoting layer disposed on the seed layer; anda laminated layer disposed on the growth promoting layer, the laminatedlayer having intrinsic perpendicular magnetic anisotropy (PMA), thelaminated layer having a composition represented by(A1/A2)_(n)/Co/Ru/Co/(A1/A2)_(p), wherein A1 is a first metal or alloy,wherein A2 is a second metal or alloy different from the first metal oralloy, and wherein “n” and “p” are a number of laminates in (Al/A2)_(n)and (Al/A2)_(p) stacks, respectively.
 2. The magnetic element of claim1, wherein the first material is selected from the group consisting ofZr, Nb, TaN, ZrN, NbN, and combinations thereof, and wherein the secondmaterial is selected from the group consisting of Mg, Sr, Si, MgZr,MgNb, and combinations thereof.
 3. The magnetic element of claim 2,further comprising a transition layer interposed between the laminatedlayer and the tunnel barrier layer, the transition layer including CoFe,CoFeB, Co, or a combination thereof.
 4. The magnetic element of claim 1,wherein the growth promoting layer includes NiCr.
 5. The magneticelement of claim 4, wherein the growth promoting layer further includesa third material, the third material being selected from the groupconsisting of Si, V, Fe, Mg, Zr, Nb, Ta, Ti, Cu, Mo, and combinationsthereof.
 6. The magnetic element of claim 1, wherein the first metal oralloy is selected from the group consisting of Co, Ni, Fe, andcombinations thereof.
 7. The magnetic element of claim 1, wherein thesecond metal or alloy is selected from the group consisting of Co, Fe,Ni, Ru, Si, V, Rh, Ir, Mg, NiCr, Pt, Pd, and combinations thereof. 8.The magnetic element of claim 1, wherein the seed layer has a thicknessfrom about 10 Angstroms to about 100 Angstroms.
 9. The magnetic elementof claim 1, wherein the growth promoting layer has a thickness fromabout 10 Angstroms to about 100 Angstroms.
 10. The magnetic element ofclaim 1, wherein the laminated layer is a free layer in a top spin valveconfiguration.
 11. The magnetic element of claim 1, wherein thelaminated layer is a reference layer in a bottom spin valveconfiguration.
 12. A magnetic tunnel junction (MTJ), comprising: a firstseed layer including a first material and a second material; a firstgrowth promoting layer disposed on the first seed layer, the firstgrowth promoting layer including NiCr or an alloy thereof; a laminatedreference layer disposed on the first growth promoting layer, thelaminated reference layer having intrinsic perpendicular magneticanisotropy (PMA); a tunnel barrier layer disposed on the laminatedreference layer; a free layer disposed on the tunnel barrier layer; asecond seed layer disposed on the free layer, the second seed layerincluding a third material and a fourth material, the first materialbeing selected from the group consisting of Zr, Nb, TaN, ZrN, NbN, andcombinations thereof, the second material being selected from the groupconsisting of Mg, Sr, Al, Si, MgZr, MgNb, and combinations thereof, thethird material being selected from the group consisting of Ta, Zr, Nb,TaN, ZrN, NbN, Ru, and combinations thereof, the fourth material beingselected from the group consisting of Mg, Sr, Ti, Al, V, Hf, B, Si,MgZr, MgNb, and combinations thereof; a second growth promoting layerdisposed on the second seed layer, the second growth promoting layerincluding NiCr or an alloy of NiCr; and a dipole layer disposed on thesecond growth promoting layer, the dipole layer having intrinsic PMA.13. The MTJ of claim 12, wherein the laminated reference layer anddipole layer have (A1/A2)_(n) and (A1/A2)_(q) structures, respectively,wherein A1 is a first metal or alloy, wherein A2 is a second metal oralloy, and wherein “n” and “q” are a number of laminates in the(A1/A2)_(n) and (A1/A2)_(q) structures, respectively.
 14. The MTJ ofclaim 13, wherein the first metal or alloy is selected from the groupconsisting of Co, Ni, Fe, and combinations thereof, and wherein thesecond metal or alloy is selected from the group consisting of Co, Fe,Ni, Ru, Si, V, Rh, Ir, Mg, NiCr, Pt, Pd, and combinations thereof. 15.The MTJ of claim 13, wherein each of “n” and “q” are from 2 to
 30. 16.The MTJ of claim 13, wherein the first metal or alloy is doped withboron up to about 50 atomic %.
 17. The MTJ of claim 12, wherein the eachof the first growth promoting layer and the second growth promotinglayer has a thickness from about 10 Angstroms to about 100 Angstroms.18. The MTJ of claim 12, wherein the each of the first growth promotinglayer and the second growth promoting layer includes an alloy of NiCr,the alloy of NiCr including a fifth material selected from the groupconsisting of Si, V, Fe, Mg, Zr, Nb, Ta, Ti, Cu, Mo, and combinationsthereof.
 19. A magnetic element, comprising: a single seed layer thatincluding a first material and a second material, the first materialbeing selected from the group consisting of Ta, Zr, Nb, TaN, ZrN, NbN,Ru, and combinations thereof, the second material being selected fromthe group consisting of Mg, Sr, Ti, Al, V, Hf, B, Si, MgZr, MgNb, andcombinations thereof; a growth promoting layer physically contacting atop surface of the single seed layer, the growth promoting layerincluding NiCr; a laminated layer disposed over the growth promotinglayer, the laminated layer having intrinsic perpendicular magneticanisotropy (PMA) and a composition represented by(A1/A2)_(n)/Co/Ru/Co/(A1/A2)_(p), wherein A1 is a first metal or alloy,wherein A2 is a second metal or alloy different from the first metal oralloy, and wherein “n” and “p” are a number of laminates in (A1/A2)_(n)and (A1/A2)_(p) stacks, respectively; and a tunnel barrier layerdisposed over on the laminated layer.
 20. The magnetic element of claim19, wherein the first metal or alloy is selected from the groupconsisting of Co, Ni, Fe, and combinations thereof, and wherein thesecond metal or alloy is selected from the group consisting of Co, Fe,Ni, Ru, Si, V, Rh, Ir, Mg, NiCr, Pt, Pd, and combinations thereof.